Originally Posted by Crankycrank

Looking at how much aluminum is missing a helicoil may not be a 100% guaranteed fix. Something like a threadsert, (see below), might be better but as you've seen this can get expensive for a proper reverse threaded part even when doing it yourself and drilling out the hole it has to be exactly perpendicular to the arm which can be a little tricky in this situation. Personally, would just look for a used replacement arm on ebay or from Co-Ops or you might find a new one for cheap somewhere.